This sounds interesting. Someone willing to give it a shot and tell us how it went?
I think it could be improved by tying in reference code. Perhaps mine the PHP manuals for sample code and put it in to allow editing.
Codepad is meant for simple snippets. I don't see anything wrong with the versions that he's currently providing.
Not downvotes. :(
Could it autodetect the language using a few heuristics (or Bayesian analysis of existing submissions) instead of having the selection down the left hand side?